Software piracy, the illegal copying of commercial computer programs, is a major problem in the personal computer industry. Software developers currently have three common techniques to prevent illegal copying of their software. The first is the use of hardware "keys". These keys are devices that attach to a computer's printer port. Before starting, the software attempts to query the key. If there is no response, or an incorrect response, the software will not start. The second technique is the use of a key disk. This is like a hardware key, except the key is a floppy disk that must be in the computer's disk drive. A third technique is to sell copy-protected software. In this technique, the developer puts the program on a floppy disk in such a form that a direct copy of the disk will produce an incorrect copy of the software.
All of the above mentioned techniques have problems. They require developers to incorporate piracy protection into their programs. They are brittle, meaning that once a single copy with the protection circumvented exists, the protection is useless. Also, they cause inconvenience to legitimate users of the software. An ideal protection scheme would make it impossible for people to use commercial software illegally, without posing any inconvenience to the developer or the user of the software.